This invention relates to a coupled lens assembly having substantially no optical axis shifting and no bonding failure between lens units constituting the coupled lens assembly.
A coupled lens assembly composed of a plurality of lenses bonded together has been widely utilized for, for example, a projection device of a projection type television.
Such a coupled lens assembly is usually formed by bonding, through intermediate bonding agent layers, a plurality of glass lenses or plastic lenses having different refractive indexes from each other. Particularly, a coupled lens assembly formed by the plastic lenses has a light weight as a whole in comparison with a coupled lens assembly formed by the glass lenses, and moreover, each of the plastic lenses has degree of molding freedom in its nature. Furthermore, the plastic lens can be easily formed to an aspherical lens, which is extremely difficult to be formed by the glass lens. In view of these facts and advantages of the plastic lenses, recently, the usage thereof has been widely developed.
In a typical coupled lens assembly of the character described above, there is provided one formed by bonding a concave lens made of, for example, polycarbonate, a convex lens made of, for example, acrylate resin and a bonding agent layer interposed between these concave and convex lens. The concave lens has a central recessed lens portion and a peripheral flat surface portion surrounding the central lens portion, and the convex lens also has a central protruded lens portion and a peripheral flat surface portion surrounding the central lens portion. In such conventional coupled lens assembly, the protruded lens portion of the convex lens is fitted into the recessed lens portion of the concave lens with the flat portions formed at peripheral portions of both the concave and convex lens portions being face to face bonded through the intermediate bonding agent layer.
With respect to such coupled lens assembly, in a case where the intermediate bonding agent layer is formed by so called a solvent type, the materials constituting the concave and convex lenses are affected or damaged by the solvent and, in another aspect, when the solvent is volatiled, the volume of the intermediate bonding agent layer is itself reduced, resulting in the change of the optical characteristic or performance of the coupled lens assembly in time elapsing, thus providing a significant problem. In view of such problem, in the prior art, there is provided an epoxy resin series bonding agent is utilized for forming the intermediate bonding agent layer.
However, the epoxy resin series bonding agent forming the intermediate layer of the conventional coupled lens assembly has a cure shrinkage coefficient of about 5 to 8% and in comparison with the solvent type bonding agent, the shrinkage after the curing is remarkably small, but in an optical view point, the lens surface is deformed after the curing, thus affecting on the initial optical characteristic or performance.
In another view point, since the peripheral portions of both the concave and convex lenses are formed to be flat, it is difficult to align optical axes of both the lenses at a time of bonding these lenses with high performance, which requires a troublesome working for making align the optical axes of these lenses. Moreover, even after once making alignment, both the optical axes are sometime shifted. When a lot of the bonding agent is used for bonding these lenses, an excessive amount of the bonding agent is flew outward through the flat surface portion between both the lenses, making worse the workability at the assembling of the coupled lens assembly. For this reason, there is a problem for using sufficient amount of the bonding agent and when the bonding agent is used in less amount, there may cause a case where air bubbles are formed inside the bonding agent layer, resulting in a bonding failure and hence not achieving a desired optical characteristic or performance.